THE OVERSHOOT EFFECT IN TRANSIENT ELECTROLUMINESCENCE FROM ORGANIC BILAYER LIGHT-EMITTING-DIODES - EXPERIMENT AND THEORY

Transient electroluminescence (EL) from organic bilayer light emitting diodes addressed by a rectangular voltage pulse often features an ove rshoot when the voltage is switched off, Experimental results are pres ented for a variety of hole transporting layers in contact with an oxa diazole layer simultaneously acting both as a blockade for holes and a s an electron transport layer. The overshoot occurs in spin coated yet not in vapor deposited samples. A model is developed to rationalize c harge recombination under the premise (i) of an interfacial layer of f inite thickness between hole and electron transport layers in which bo th transport molecules coexist and (ii) of interfacial energy barriers impeding both hole and electron passage. It predicts the occurrence o f an EL overshoot due to the recombination of stored electrons and hol es under the action of their mutual space charge field when the extern al voltage is switched off. The temporal pattern of the predicted tran sient EL signal is in good agreement with experiment. (C) 1997 America n Institute of Physics.